The invention herein relates to processes for melting refractory oxides in electric furnaces. More particularly it relates to a method for reducing or preventing the oxidative deterioration of refractory metal electrodes and orifices in such furnaces.
For a number of years electric melt resistance furnaces have been used to melt refractory inorganic oxides, commonly silica and alumina. These molten oxides can then be spun or blown into attenuated fibers which are used as thermal insulations. Those fibers which have relatively lower melting points are commonly referred to as "glass fibers" while those with relatively higher melting points are referred to as "refractory fibers." The glass fibers are used as thermal insulation for homes, office buildings and the like, while the refractory fibers are used as thermal insulation for furnaces, kilns and similar devices. These various types of fibers have melting points ranging from about 1000.degree. F to about 3300.degree. F or higher (about 540.degree. C to about 1815.degree. C or higher). There is no absolute line of demarcation between the fibers commonly termed "glass" and those commonly termed "refractory," but normally the latter are considered to be those intended for use at temperatures above about 1200.degree. F (650.degree. C).
Electric furnaces for melting such oxides commonly have the general shape of a bowl with a drain orifice in the center. Normally three or more electrodes are positioned around the orifice. The refractory oxides to be melted are charged, usually in powdered or granulated form, into the bowl shaped furnace such that they cover the orifice and the electrodes. After start-up using a high temperature flame to form an initial molten pool of oxides, electrical current passing through the electrodes and the molten oxides causes resistance heating of the oxides and in turn continued melting of the oxide materials. As the oxides melt they drain through the refractory metal orifice into fiber forming apparatus positioned below. In the meantime raw materials in the form of oxide powders are continually added to the furnace and in turn melted for fiberizing, thus resulting in continuous formation of an oxide melt and subsequent fibers. Frequently a melting and fiberizing operation, once started, will be run continuously for periods of many days, weeks of months. This means that the electrodes must operate satisfactorily for such periods, for once the melting operation has started there is often no opportunity to replace or repair the electrodes without shutting down the furnace. Even with those furnaces where electrodes can be replaced while the furnace remains in service, such replacement is both difficult and costly.
Similarly, the orifice must remain substantially unchanged in shape or dimensions during the entire run, for the flow of molten oxide into the fiberizing unit is controlled by this orifice. The orifice is formed with a specific and carefully calculated diameter, designed such that a closely controlled predetermined quantity of molten oxide drains down to the fiberizing unit located below the furnace. The quantity and especially the quality of the fiber formed is directly related to the flow rate of the oxide melt from the furnace through the orifice to the fiberizing unit. Control of such flow rate is maintained by a needle-like flow controller which operates in cooperation with the orifice to control the quantity of oxide flowing through the orifice. If the orifice deteriorates, it becomes substantially enlarged and can no longer cooperate with the "needle valve" control apparatus to regulate the flow of oxide melt through the orifice. The fiberizing unit thus becomes essentially "flooded" with excess oxide melt and can no longer function to produce satisfactory fiber. This deterioration of the orifice is more critical to the operation than the deterioration of the electrodes, for in some cases the electrodes can be moved and/or the electric current flow can be adjusted so as to compensate to some extent for the deterioration of the electrodes. However, the fixed orifice cannot readily be adjusted to compensate for the removal of the refractory orifice metal and resulting increase in the orifice diameter.
As noted, electric furnaces have been used successfully in the past to form "glass" melts which are primarily composed of mixtures of silica and soda, and "refractory" melts which are primarily composed of mixtures of silica and alumina. Frequently small amounts of other oxides, such as titania, zirconia, iron oxides or the like may be present, particularly with the lower temperature "glass fibers." Heretofore it has not been observed that any of these materials has had any unduly detrimental effect on the operation of the electric furnaces.
In recent years, however, refractory oxide fibers containing silica and alumina and up to about 10% chromia have found widespread use. Such fibers have been found to be very effective thermal insulations at temperatures up to about 2700.degree. F (1480.degree. C). Typical of such chromia containing fibers are those sold under the trademark CERACHROME by the Johns-Manville Corporation and described in U.S. Pat. No. 3,449,137 to Ekdahl.
Initially the chromia containing oxide melts were made in carbon arc furnaces for fiberization. Success of refractory metal electrode electric furnaces with other oxide melts led to attempts to form chromia containing melts in such furnaces. However, it was discovered that when melts containing approximately 0.5% to 10% or more by weight of chromia were used in electric furnaces, the refractory electrodes and orifices rapidly deteriorated and wore away. In some instances electrodes and orifices which would have served for a number of months with other types of oxide melts deteriorated in the presence of chromia so rapidly that the furnaces had to be shut down in a matter of a few hours or days. Prior to the invention herein, all such attempts to melt chromia containing oxides in electric furnaces with refractory metal electrodes and orifices have resulted in rapid deterioration of the electrodes and orifices and have been considered totally unsuccessful.
It is therefore an object of this invention to provide a method for permitting the melting of chromia containing oxide mixtures in electric furnaces in the presence of refractory metal electrodes and orifices (such as molybdenum) while still providing a satisfactory service life of the orifice and electrode components.